(i) Field of the Invention
This invention relates to an electronic heat-exchanger in the form of a novel device for providing heated and humidified air from a flow of air, passing a heat source, which device is especially useful for inhalation warming of hypothermia victims.
(ii) Description of the Prior Art
Hypothermia is one of the most frequently encountered and yet often overlooked aspects of emergency medicine. Hypothermia, like other medical conditions, is graduated by the degree of severity and the symptoms and urgency of treatment may differ radically at different levels. In mild hypothermia above about 35.degree. C. (about 95.degree. F.), the accepted treatment is passive rewarming by natural or endogenous heat generation, which is simple, requires no equipment (other than a warm environment and/or blanket), and has no inherent morbidity. Moderate hypothermia occurs in the range of about 32.degree. to about 35.degree. C. (about 89.6.degree. to about 95.degree. F.) and may produce loss of motor control, slurred speech and amnesia; serious hypothermia occurs at body core temperatures below about 32.degree. C. (about 89.6.degree. F.) and is indicated by muscle rigidity, peripheral cyanosis and shock; and severe hypothermia occurs at temperatures from about 25.degree. to about 28.degree. C. (about 77.degree. to about 82.4.degree. F.) at which the victim may have lost deep tendon reflexes and suffer ventricular fibrillation and may appear dead with no palpable pulse or audible heartbeat. Body temperatures below about 25.degree. C. (about 77.degree. F.) cause cardio-pulmonary arrest and death. These forms of hypothermia clearly present life-threatening medical problems complicating the treatment of the victim.
It is becoming established that the safest and most efficient technique for treating the severely hypothermic victim is by active core rewarming, i.e. the delivery of heat primarily to the body core or central circulation system (and also avoiding simultaneous rapid rewarming of the skin and extremities). Of several methods, only inhalation rewarming is suitable for use by paramedics and other trained emergency rescue personnel at a rescue site or during transport to hospital or clinical facilities.
Warm, moist air is also very useful for relieving laryngectomy and tracheotomy patients, and to relieve asthmatic bronchial spasms.
In the normal breathing process, inhaled air becomes warmed and humidifies as it passes through the nasal, tracheal and bronchial passages. This basic body function protects the delicate membranes in the lungs, but may not be sufficiently effective in heavy or rapid breathing of very cold dry air. During exhalation, some heat and moisture is returned to the walls of the breathing passages, but most of the heat energy and moisture is lost in the exhaled gases.
At rest and at a comfortable room temperature, the energy loss is of the order of about 1 Kcal per hour, and is easily compensated by normal body heat production. However, at low temperatures and high altitudes, the energy loss could be about 230 Kcal and about 250 grams of water per hour. This is a significant portion of the energy output of the body and contributes to the harmful effects caused by the inhalation of extremely cold air. In the absence of the ingestion of food, the mere use of warm clothing may not be sufficient to retain a desirable amount of the energy. Also, since thirst response is suppressed by extreme cold, desiccation could become a problem.
There is thus a widespread need for apparatus capable of producing heated and moist air, and in most cases such need is for air heating apparatus which is self-contained and not requiring large amounts of electric power, and which is compact, safe and not needing a flame to heat the air, and lightweight so as to be conveniently portable. As discussed above, one important current need for such air heating apparatus is to produce heated air to be inspired by persons through a suitable device into the lungs, which is an efficient means for warming persons suffering from body core heat loss that has reached the stage of hypothermia. Such inspiration of heated air directly into the airways and lungs is the most effective and safe way to stabilize and rewarm a person, and to bring hospital-type treatment to the rescue situation.
A number of devices have been developed in the prior art for the purpose of reducing the harmful or dangerous effects caused by the inhalation of extremely cold air. Many patented devices and techniques for heating and humidifying breathing gases have been devised for many purposes.
It is well known in inhalation therapy that gases which are to be delivered to a subject or patient should be humidified and warmed prior to inhalation by the subject. Exemplary of the type of gases delivered to a patient include oxygen as well as a mixture of air and oxygen.
Various types of systems have been proposed for generating humidifying gases, generally for use on connection with respiratory care systems. For example, one type of humidification unit includes a chamber suitable for containing a quantity of water, the bottom wall thereof having a heater plate associated therewith. An aluminized insert element is provided which is constructed in the form of a spiral within the chamber. Between the concentric loops of the spiral a sheet of absorbent paper is positioned in loose fitting relationship, the absorbent paper being manually fed into the spiral until the same is present between all of the concentric loops of the spiral. The aluminum element functions to conduct heat from the lower heater plate upwardly while the absorbent paper functions to take water up by capillary action thereby to provide moisture throughout the spiral assembly. A gas is injected into the unit and must be specifically directed to the open end of the spiral with the necessity that the gas completely pass through the open concentric circles of the spiral and exit from the innermost portion of the spiral upwardly to a gas outlet and from there into an outlet tube for delivery to a subject.
U.S. Pat. No. 3,954,920 patented May 4, 1976 by W. I. Heath provided a gas humidification system including a substantially closed chamber formed by a plurality of side walls, a bottom wall and a top wall having a gas inlet and outlet ports associated therewith, and heat means associated with the bottom wall. A humidification element was removably positionable within the chamber. That element included a heat conductive metallic member having an end for contact with the heat means and extending upwardly therefrom, and a layer of water absorbent material fixedly secured to at least one of the surfaces of the metallic member. The metallic member presented a multi-faceted surface and the absorbent material paralleled the multi-faceted surface of the body portion of the metallic member thereby presenting an increase surface area for gas to pass over and around the humidification element at elevated temperatures. This was said to increase the relative humidity of the gas prior to inhalation by a patient.
U.S. Pat. No. 3,912,795 patented Oct. 14, 1975 by R. R. Jackson provides an apparatus for humidifying a gas and delivering it to be breathed. The apparatus included a gas chamber having an inlet for communication with a source of the gas, an outlet, and a water chamber coextensive over a zone with a portion of the gas chamber. One of the chambers had a wall which was permeable to water vapour at the zone. Water was brought into intimate contact with one side of the wall at the zone at a vapour pressure sufficient to cause passage of water vapour through the wall and into the gas chamber to humidify gas therein. The apparatus included water return and supply tubes effectively sealed to the water chamber and arranged for respective connection to a source of vacuum and to a water reservoir exposed to atmospheric pressure.
U.S. Pat. No. 4,026,285 patented May 31, 1977 by R. R. Jackson provided a humidifier for air or gas flow to the lungs to heat the air flow and to saturate it with water vapour. Heated water injected into a humidifying passage flows down multiple paths in the gas stream. Heating of the water is done on a localized basis at sterilizing temperature while over-all temperature is kept lower using a heater combined with the pump of the injection line. All surfaces of the humidifier are exposed to the water. The gas outlet connection communicates with a space between the water paths and the reservoir, and is directed toward the reservoir. The humidifying passage is a tubular column holding a mass of tangled inert filaments.
U.S. Pat. No. 4,038,980 patented Aug. 2, 1977 by I. Fodor provided an air humidifier for a surgical breathing machine which had an evaporating chamber containing a temperature regulated low thermal mass heater. A water drip feed unit dripped water onto a sheath of porous material of the heater at a predetermined rate. The water was substantially completely and instantaneously evaporated to create an air temperature and relative humidity within predetermined limits. The heater temperature and water supply rate had manual adjustment means coupled together so that such predetermined limits were not exceeded. The evaporating chamber was tubular and was of similar cross section to inter- connecting conduits so as not unduly to interfere with pressure waves from the breathing machine.